The production and characterization of nano cellulose e.g. certain types of microfibrillated cellulose from wood fibers was described by Turbak et al. [1] and Herrick et al. [2] in the 1980s. Nano cellulose was obtained by forcing a cellulose fibers suspension through a mechanical communition device at high pressure. The procedure resulted in the production of gel-like water dispersions with a nano cellulose concentration of 1% w/w, or lower, with high aspect ratio [1-3].
Because of the unique characteristics of nano cellulose fibrills, such as a very high specific surface area, and the formation of a highly porous gel-like network, an extensive amount of research has been conducted on nano cellulose in the last 30 years. Several recent publications demonstrate how nano cellulose can be used for various purposes within the materials field, such as constituting reinforcement in nanocomposites dispersion stabilizers, constituting antimicrobial films, filtration media, and oxygen barrier material in food and pharmaceutical applications [4, 5].
WO2010/003860 discloses liquid cleansing compositions comprising nano cellulose and surfactant, but does not relate to coatings.
US2012/0205065 discloses addition of aqueous nano cellulose to paper stock for the preparation of paper with high dry strength. The document does not relate to coatings.
US 2010/0272980 relates to a fiber composite for a substrate. A dispersion of microfibrillated cellulose fibers is disclosed, wherein the dispersion medium may be water, and the dispersion medium may further comprise a surfactant.
WO 2008008576 discloses a water-based formulation comprising a water-based polymer and a fiber, which can be micro fibrillated cellulose fiber. The formulation may also comprise surfactants.
US2011/0081554 discloses a nano cellulose coating composition comprising 1-90% nano cellulose dispersion, but does not disclose use of any surfactant. Likewise, WO2011/056130 discloses the use of nano cellulose in a coating, but does not disclose the use of any surfactant.
Low water and gas permeability is crucial for a lot of food packaging and low oxygen transmission is one of the main requirements. To meet this demand paper based packaging are laminated with aluminium or extrusion coated with synthetic polymers such as ethyl vinyl alcohol (EVOH) in one or several thin layers.
An emerging trend in the packaging sector is the creation of a barrier on packaging paper and packaging board, by the use of biodegradable coating(s). The oxygen barrier property of nano cellulose is of particular interest for paper based packaging, and opens up new packaging solutions based on renewable and sustainable sources.
Nano cellulose can be applied as a coating with paper or paperboard as substrate, due to its gel-like behaviour, its chemical compatibility with the substrate and the possibility of dispersing nano cellulose in water. The nano cellulose coating applications, as compared with lamination applications, have the advantages of good adhesion and no need for adhesive, which implies cost reduction.
However, even though the rheological characteristics of nano cellulose suspensions are fundamental for possible utilization thereof in the materials and food industries (e.g. in coatings, as thickening agent, in extrusion processes), little has been published. Herrick et al. [2], who first studied the rheological behaviour of nano cellulose, discovered a pseudo-plastic (shear thinning) behaviour of a 2% water dispersion of nano cellulose.
An article by Iotti et al. (2010) [6] studied nano cellulose water dispersions at various concentrations (1-4% w/w) at temperatures between 25 and 60° C. Nano cellulose was shown to form gels at low concentration in water and exhibit a shear thinning behaviour with decreased viscosity, on shearing.
Coatings of a paper-based substrate with a nano cellulose dispersion are known to exhibit excellent properties with respect to low oxygen permeation and are also environmentally friendly, being cellulose-based.
JP58191296 discloses a coating composition comprising an aqueous dispersion of vinyl polymer and/or resin containing micro-fibrillated cellulose. The composition may also comprise a surfactant.
In an article by Xhanari [7] is described the use of the surfactant CTAB in combination with nanofibrillated cellulose or microfibrillated cellulose. The concentration of the fiber suspensions before fibrillation for two batches was 0.4% w/v.
Due to the properties of nano cellulose, nano cellulose dispersions are very difficult to concentrate without obtaining aggregates of the fibrills. Furthermore, concentrated nano cellulose dispersions commonly have a high viscosity making them difficult to handle industrially.
The high network forming capability of nano cellulose water dispersions is dependent on the temperature and the nano cellulose concentration. The viscosity decreases with increasing temperature and increases with increasing nano cellulose concentration [6].
The advent of nano cellulose fibril aggregation at higher nano cellulose concentration levels makes it a challenge to obtain high viscosity and low concentration nano cellulose dispersions.
Problems of high viscosity and low concentration are present in industry and there is a strong need for highly concentrated nano cellulose water-based coatings and a process for their use, to allow for lower energy consumption in drying process steps.